A capacitor is a passive electronic component that is used to store energy in the form of an electrostatic field, and comprises a pair of electrodes separated by a dielectric layer. When a potential difference exists between two electrodes, an electric field is present in the dielectric layer. An ideal capacitor is characterized by a single constant value of capacitance. This is a ratio of the electric charge on each electrode to the potential difference between them. In practice, the dielectric layer between electrodes passes a small amount of leakage current. Electrodes and leads introduce an equivalent series resistance, and dielectric layer has limitation to an electric field strength which results in a breakdown voltage. The simplest energy storage device consists of two parallel electrodes separated by a dielectric layer of permittivity ε, each of the electrodes has an area S and is placed on a distance d from each other. Electrodes are considered to extend uniformly over an area S, and a surface charge density can be expressed by the equation: ±ρ=±Q/S. As the width of the electrodes is much greater than the separation (distance) d, an electrical field near the centre of the capacitor will be uniform with the magnitude E=ρ/ε. Voltage is defined as a line integral of the electric field between electrodes. An ideal capacitor is characterized by a constant capacitance C, defined by the formula (1)C=Q/V,  (1)which shows that capacitance increases with area and decreases with distance. Therefore the capacitance is largest in devices made of materials of high permittivity.
A characteristic electric field known as the breakdown strength Ebd, is an electric field in which the dielectric layer in a capacitor becomes conductive. The voltage at which this occurs is called the breakdown voltage of the device, and is given by the product of dielectric strength and separation between the electrodes,Vbd=Ebdd  (2)
The maximal volumetric energy density stored in the capacitor is limited by the value proportional to ˜ε·E2bd, where ε is dielectric permittivity and Ebd is breakdown strength. Thus, in order to increase the stored energy of the capacitor it is necessary to increase dielectric permeability ε and breakdown strength Ebd of the dielectric.
Breakdown of the dielectric layer usually occurs when the intensity of the electric field becomes high enough to “pull” electrons from atoms of the energy storage molecular material and make them conduct an electric current from one electrode to another. Presence of impurities in the energy storage molecular material or imperfections of the crystal dielectric layer can result in an avalanche breakdown as observed in capacitor.
Other important characteristic of an energy storage molecular material is its dielectric permittivity. Different types of energy storage molecular materials are used for capacitors and include ceramics, polymer film, paper, and electrolytic capacitors of different kinds. The most widely used film materials are polypropylene and polyester. Increase of dielectric permittivity allows increasing of volumetric energy density which makes it an important technical task.
An ultra-high dielectric constant composite of polyaniline, PANI-DBSA/PAA, was synthesized using in situ polymerization of aniline in an aqueous dispersion of poly-acrylic acid (PAA) in the presence of dodecylbenzene sulfonate (DBSA) (see, Chao-Hsien Hoa et al., “High dielectric constant polyaniline/poly(acrylic acid) composites prepared by in situ polymerization”, Synthetic Metals 158 (2008), pp. 630-637). The water-soluble PAA served as a polymeric stabilizer, protecting the PANI particles from macroscopic aggregation. A very high dielectric constant of ca. 2.0*105 (at 1 kHz) was obtained for the composite containing 30% PANI by weight. Influence of the PANI content on the morphological, dielectric and electrical properties of the composites was investigated. Frequency dependence of dielectric permittivity, dielectric loss, loss tangent and electric modulus were analyzed in the frequency range from 0.5 kHz to 10 MHz. SEM micrograph revealed that composites with high PANI content (i.e., 20 wt %) consisted of numerous nano-scale PANI particles that were evenly distributed within the PAA matrix. High dielectric constants were attributed to the sum of the small capacitors of the PANI particles. The drawback of this material is a possible occurrence of percolation and formation of at least one continuous conductive path under electric field with probability of such an event increasing with an increase of the electric field. When at least one continuous path (track) through the neighboring conducting PANI particles is formed between electrodes of the capacitor, it makes a breakdown voltage of such a capacitor being relatively low.
Single crystals of doped aniline oligomers are produced via a simple solution-based self-assembly method (see, Yue Wang, et. al., “Morphological and Dimensional Control via Hierarchical Assembly of Doped Oligoaniline Single Crystals”, J. Am. Chem. Soc. 2012, 134, pp. 9251-9262). Detailed mechanistic studies reveal that crystals of different morphologies and dimensions can be produced by a “bottom-up” hierarchical assembly where structures such as one-dimensional (1-D) nanofibers can be aggregated into higher order architectures. A large variety of crystalline nanostructures, including 1-D nanofibers and nanowires, 2-D nanoribbons and nanosheets, 3-D nanoplates, stacked sheets, nanoflowers, porous networks, hollow spheres, and twisted coils, can be obtained by controlling the nucleation of the crystals and the non-covalent interactions between the doped oligomers. These nanoscale crystals exhibit enhanced conductivity compared to their bulk counterparts as well as interesting structure—property relationships such as shape-dependent crystallinity. Furthermore, the morphology and dimension of these structures can be largely rationalized and predicted by monitoring molecule—solvent interactions via absorption studies. Using doped tetra-aniline as a model system, the results and strategies presented in this article provide insight into the general scheme of shape and size control for organic materials.
Capacitors as energy storage device have well-known advantages versus electrochemical energy storage, e.g. a battery. Compared to batteries, capacitors are able to store energy with very high power density, i.e. charge/recharge rates, have long shelf life with little degradation, and can be charged and discharged (cycled) hundreds of thousands or millions of times.
However, capacitors often do not store energy in such little volume or weight as in a battery, or at low cost per energy stored, making capacitors impractical for applications such as in electric vehicles. Accordingly, it would be an advance in energy storage technology to provide storing energy more densely per volume and/or mass.
Aspects of the present disclosure provide solutions to the problem of the further increase of volumetric and mass density of reserved energy of the energy storage device, and at the same time reduces cost of materials and manufacturing process.